Not applicable.
Not applicable.
(1) Field of the Invention
The present invention generally relates to the manufacturing and testing of composite articles. More particularly, this invention relates to a method and system for assembling and nondestructively detecting and characterizing flaws in an assembly that includes a composite component.
(2) Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel and cobalt-base superalloys. Yet as higher temperatures are required for gas turbine engines, alternative materials have been proposed. Materials containing silicon, particularly ceramic matrix composite (CMC) materials with silicon carbide (SiC) as a matrix material and/or as a reinforcing material, are currently being considered for high temperature applications, such as combustor and other hot section components of gas turbine engines.
In the effort to develop CMC composites for gas turbine engine applications, there is a need to be able to assemble composite components with other structures, and nondestructively detect and characterize any processing or operational flaws below the component surface as well as any assembly flaws. Due to their composite nature and the relatively low atomic numbers of the elements from which CMC""s are formed, traditional nondestructive methods such as ultrasonic and x-ray testing are not fully satisfactory. Though ultrasonic testing has been used to detect and characterize flaws in composite materials, it cannot be easily used to distinguish between small flaws in a CMC component and fluctuations due to modulus and density variations that are inherent in CMC components. While effective for detecting and evaluating volumetric flaws in CMC composites, x-ray detection methods have not been very effective when used on low atomic number materials. Furthermore, x-ray detection methods have not been effective in detecting delaminations, which are a relatively common type of flaw in CMC components.
While infrared (IR) detection methods have been successfully used to detect and estimate the depth of delaminations in composites, local thermal conductivity variations in CMC composite materials can mimic local delaminations in certain situations. In addition, the ability of IR detection methods to estimate flaw depth is limited by the thermal radiation of the surrounding material, which can result in a situation in which a small signal must be detected in a large and xe2x80x9cnoisyxe2x80x9d thermal background. At some levels of sensitivity, it becomes difficult to distinguish between a small flaw and ordinary variations in a composite material. Recent work reported by L. D. Favro et al. in xe2x80x9cInfrared Imaging of Defects Heated by a Sonic Pulse,xe2x80x9d Review of Scientific Instruments, Vol. 71, No. 6 (June 2000), involves using IR imaging to detect localized frictional heating generated at subsurface defects by sonic excitation. However, Favro et al. describe using short pulses (50 to 200 ms) of high frequency (e.g., 20 to 40 kHz) applied to limited points on the surface of the object being inspected, which can be impractical or cumbersome when the object to be examined is large and/or an assembly of components whose various interfaces can be the location for flaws.
In view of the above, it would be desirable if an improved method were available for nondestructively detecting and characterizing flaws in a composite article, such as CMC articles, particularly with respect to detecting assembly defects and delaminations within such articles.
The present invention provides a method and system for nondestructively detecting and characterizing defects in articles, and particular assemblies that may include a component formed of a composite material. The method generally entails vibrating an entire article to induce an oscillating strain in the article so that any flaws in the article are heated by localized friction, as would occur between opposing surface portions of a defect or an interface between components of an assembly. The oscillating strain can be induced with a variety of known methods, such as using a piezoelectric element or another suitable device. The article is then infrared imaged to detect any localized temperature rises in the article associated with the localized friction at a defect.
From the above, one can appreciate that the method and system of this invention are able to detect and characterize flaws nondestructively in composite materials and assemblies comprising a composite article. A particular advantage of the invention is that thermal signatures generated from tight flaws are quite large compared to the thermal background of a composite article being examined, thereby significantly improving the capability of detecting even relatively small flaws and defects. These enhanced flaw signatures are conducted to the surface of the article, where they can be detected by infrared imaging device.
Another feature of the method and system of this invention is the ability to detect inadequate clamping loads generated by fasteners that secure components of an assembly together, which when subject to the oscillating strain causes frictional heating at the interfaces between the mating surfaces of the components. By detecting fasteners with low clamping loads, such fasteners can be tightened or replaced as necessary, and/or the installation of fasteners of subsequently produced assemblies can be modified to achieve adequate clamping loads. If one of the components is formed of a composite material, another benefit of the invention is that the vibrating step causes projecting features typically present on the surfaces of composite component to be reduced by wear or grinding. As such, the method can be employed to improve the interface of a composite component with a second component of the assembly, after which the fasteners that hold the components together can be tightened or replaced as necessary. While particularly well suited for use on composite materials and assemblies comprising composite articles, the test method and system of this invention could be adapted for use with other materials and assemblies, such as to detect welding flaws in metals.